Field of the Invention
The invention relates to a dentists' preparation instrument having a turbine for driving a tool by means of compressed air. A rotor arranged in a turbine compartment and rotating about a longitudinal axis has blades extending to a face of the rotor. The turbine compartment has a wall opposite the face and has a discharge port for the compressed air to a return air duct, wherein the discharge port is arranged in the turbine compartment in such a way that at least parts of the blades of the rotor pass by the discharge port in rotation about the longitudinal axis.
Description of the Related Art
In known dentists' preparation instruments having a turbine drive, also known as dental turbines, compressed air is used to drive a rotor. The kinetic energy of the air causes an impulse on the blades of the rotor through an exchange of momentum. From this impulse, a torque is created on the rotor, and the resulting rotational speed depends primarily on the speed of the air flowing out of the nozzle during idling. At the idling speed, i.e., during no-load operation of the turbine, the torque and thus the usable power approach zero while the maximum power is established at half the idling speed in a characteristic manner. However, increasing the idling speed to increase the maximum power has a negative effect on the lifetime of the bearings and on the noise level of turbines.
DE 100 60 152 B4 describes a dental turbine hand piece in which a rotor is equipped with a first and a second turbine wheel to increase the torque, and connecting ducts are provided to deflect the pressurized medium from the first turbine wheel to the second turbine wheel. In one embodiment, the flow passes twice through the second turbine wheel, which brakes the rotor and reduces the rotational speed.
The object of the invention is to achieve a reduction in the idling speed in order to improve the lifetime and noise behavior without restricting the maximum torque or the effective power to an extent that would be of practical relevance.